These are really cool. The reason they're called touch-me-nots is that when they form seed pod, they explode when you touch them. I kid you not, they actually explode with an audible pop, though I'm not sure how.

Here's the entire plant, and it's only about 15cm tall. The flowers are less than a 1cm. The two opposite leaves are the only ones it will have. This species is actually pretty uncommon in California, but David happened to know a spot for it.